A smart phone is very convenient for outdoor use, telephone numbers of relatives and friends are stored in a phone list in the smart phone, and touching the name of relatives and friends displayed on the smart phone is used for dialing out.
After going home, generally a home phone is used for dialing out, the home phone has no phone list, so the phone list in the smart phone is used for looking up, and then dialing out through the home phone, it is not convenient.
WiFi is a wireless local area network (WLAN) system based on IEEE 802.11. The communication range of WiFi are limited, a typical wireless router based on 802.11b or 802.11g and WLAN antenna has a range of about 32 meters indoors and 95 meters outdoors, so WiFi is only suitable to home or indoor wireless communication.
VoIP (Voice over Internet Protocol) is one of the popular communication technology. In VoIP, SIP (Session Initiation Protocol) defined by IETF is the most widely used protocol because of its simple structure, expandability and easy operation.